This invention relates to a fluorescent lamp including a discharge tube bent substantially in a plane, and, more particularly, to a lamp construction in which the discharge tube is bent to a shape defining a substantial part of the boundary of a zone in the plane.
The luminous output of fluorescent lamps is defined by the mercury vapor pressure in the discharge tube among others. The pressure of the mercury vapor depends on the temperature of the cold spot in the tube which is a place where mercury condenses. Since the electrodes of fluorescent lamps generate heat, the cold spot temperature is influenced by the relative position of the electrodes with respect to the cold spot.
A fluorescent lamp including a discharge tube disposed substantially in a plane and shaped to define a substantial part of the boundary of a zone in the plane is disclosed by U.S. Pat. No. 4,458,301. The discharge tube defines the boundary including at least one straight portion and the ends of the tube are re-entrant into the zone. A lamp support housing, which is disposed within the zone, receives the ends of the tube and provides electrical connection to the electrodes. The cold spot of this type of lamps develops in exhaust tubes inserted in the discharge tube, and its temperature is highly influenced by the operating position of the lamp. The primary reason of it is that the electrodes, which are mounted into both ends of the discharge tube and develop heat while the lamp is operating, lie in the vicinity of the exhaust tubes. Consequently, the temperature of the cold spot is highly dependent on the operating position of the lamp. If the cold spot within exhaust tube is above the electrode, its temperature is higher than if it is under the electrode. Therefore, the optimum operating position of this type of lamps containing liquid mercury is a vertical electrodes up position.
The case is different with amalgam filled lamps where the necessary mercury vapor pressure is defined primarily by the composition of the amalgam and thus a luminous output is obtained which is basically independent from the operating position of the lamp. The drawback of the fluorescent lamp filled with amalgam is the longer warming up period during which the lamp produces only a part of its rated luminous output.
The luminous output of fluorescent lamps is also defined by the electric power consumed by the lamp. This power is proportional to the arc voltage of the lamp voltage which is primarily determined by the geometry and the length of the discharge arc. If a lamp with higher luminous output is needed, while the tube diameter is given, a discharge tube with longer arc length has to be made. However, a discharge tube with longer arc length implies a greater size of the lamp which is still limited by the lamp fixture.
Thus there is a particular need. to provide a fluorescent lamp including a discharge tube disposed substantially in a plane which has a cold spot independent from the operating position of the lamp as well as a higher luminous output at unchanged or smaller overall dimensions.
In an exemplary embodiment of the invention, a fluorescent lamp comprises a discharge tube disposed substantially in a plane and shaped at least in part to define a substantial portion of the boundary of a zone in the plane. The part of the tube defining the boundary includes at least one straight portion. The discharge tube has a central axis and sealed ends provided with electrodes and at least two tube sections running substantially parallel to each other. Each tube section has at least one blind-sealed end and the tube sections are connected in series through bridges in the vicinity of the blind-sealed ends to define a single continuous discharge space to be excited by electrical power supplied to the electrodes. A lamp support housing is positioned within the zone and the ends of the discharge tube as well as the blind-sealed ends of the tube sections are re-entrant into the zone. The ends of the discharge tube are received in the lamp support housing. The lamp support housing carries means suitable for mechanically and electrically connecting to a socket and includes lead-in wires connecting the electrodes directly or through an operating circuit to the means suitable for electrically connecting to a socket.
This construction has two basic advantages over the fluorescent lamp described in U.S. Pat. No. 4,458,301. One advantage is that the tube sections running parallel to each. other increase the discharge arc length significantly which results in higher lumen output at unchanged or smaller overall dimensions. Another advantage is that well-defined cold spots develop in the vicinity of the bottom portions of the blind-sealed ends since the discharge duct goes through the bridges and does not heat the bottom portions intensively. These cold spots are much farther from the lamp electrodes than the cold spots in the exhaust tubes of the lamp disclosed in the prior art patent. The heat generated by the electrodes exerts much less influence on the cold spots of the lamp provided by the present invention. This ensures cold spots independent from the operating position of the lamp.